The present invention relates to electric motors. In particular, it relates to DC motors of the type which are especially adapted to produce a relatively low speed, high torque rotation of a shaft, at relatively high levels of efficiency.
It is well known that the most efficient conventional direct current (DC) electric motors are inherently high speed. In addition, such DC motors, which usually feature moving conductors in a stationary magnetic field, are limited in terms of output torque by the minimum air gap which can be maintained between the poles of the magnets and the moving conductor, all else being equal. The conventional expression for the force on a conductor (the torque in a motor) is F=k BI (where B is the magnetic field intensity and I is the current through the conductor). Since the magnitude of the magnetic field follows the immutable physical "inverse square law", increasing the airgap disproportionately decreases the force or torque. For example, doubling the length of the air gap reduces the magnetic flux and the resulting force by a factor of four. Therefore, in conventional motor design the air gap is held to a minimum. Unfortunately, other engineering factors, such as the accumulation of mechanical tolerances, air drag losses between closely spaced moving surfaces, concomitant cost factors, and the like, force air gaps of 0.03 inches to greater than 0.10 inch.
Many electric motor usages, such as ventilation fans, blowers, vacuum cleaners, etc. function well at high rotational speeds. There exists, however, a significant number of important applications of electric motors which require relatively low speed and high torque. Some of these applications include motors for electrical powered vehicles, machine tools, electric chain saws, clothes dryers, pumps, large weapon controls, conveyor belts, paper feeds for office machines, battery-operated handtools and many others. Conventional electric motors do not efficiently operate at the low rotational speeds necessary in the above cited examples.
To solve this problem, designers commonly gear down high speed motor outputs to their eventual, low speed-high torque intended mechanical usage. Unfortunately, each gear (or equivalent belting combination) has a finite efficiency (and concomitant cost) so that the overall efficiency of the system is cumulatively degraded. As an example, a golf cart going 10 mph with driving wheels which are two feet in diameter requires a wheel speed of 140 rpm. If one were to utilize a 12,000 rpm motor, a speed reduction of about 86:1 would be necessary. Practical gear reductions are limited to about 8:1. Thus, it would require at least three sets of gears, shafts, bearings, etc. to achieve the desired speed reduction. The result is high expense (gearing often must be precision-machined), inherent motor and gear whine, and the multiplicity of parts which greatly reduces reliability and complicates maintenance.